(1) Field of the Invention:
The present invention relates to telephone trunk circuits and more particularly to a sensor circuit for use in a digital private automatic branch exchange (PABX), to detect supervisory signals between a central office and a PABX. The present circuit, designed for inclusion in a PABX trunk circuit, provides detection of incoming "call-for-service" signals in a ground start configuration and detects continuity of the loop between the central office and the PABX trunk circuit.
(2) Description of the Prior Art:
In a telephone system it is necessary that supervisory signals be sent via a trunk circuit from a PABX to a central office and from a central office to a PABX. For example, one of these signals indicates a request to be served or "call-for-service" from a central office to a PABX. This signaling is accomplished by applying an earth ground to the tip lead of the trunk circuit at the central office.
Existing trunk circuits typically sense the application of such supervisory signals by the use of polar relays. Such polar relays are expensive, require relatively large amounts of space and are generally of lower impedance than desirable. Polar relays are generally unusable for trunk circuits and electronic digital PABX's because of their size, low impedance, and slow speed of operation.
A partial solution to these problems is taught by U.S. Pat. No. 3,849,605, issued Nov. 19, 1974, to Stanley L. Russell. This patent demonstrates the use a high voltage bridge rectifier circuit, and an optical coupler including a light emitting diode and photo-sensitive transistor. The circuit described therein teaches only a continuity sensor. Furthermore, the solid state continuity check circuit described in the Russell patent must be switched into and out of the tip and ring loop by the operation of switching relays in order to avoid distortion during voice transmission. This solution is undesirable because it exhibits a low bridging impedance and must be removed from the loop for transmission purposes and secondly because it is dependent upon the slower operation of relay contacts.
Another solution to these problems is taught by U.S. Pat. No. 4,190,745, issued Feb. 26, 1980, to Julius Jusinskas, Jr., et al. This circuit teaches "call-for-service" detection and continuity sensing through the use of a full wave bridge rectifier and series connected resistances bridged across the central office loop. The full wave bridge rectifier output feeds plural optical couplers which derive the needed signals. Such a method is undesirable in that it lacks immunity from 60 hertz induced longitudinal voltages in the supervisory mode and introduces excessive longitudinally induced 60 hertz noise onto the transmission pair in the transmission mode. Further this circuit requires the use of non-standard optical couplers and actively trimmed resistors in order to operate within its design requirements. Accordingly, it is the object of the present invention to provide a new and improved "call-for-service" and continuity sensor circuit that overcomes the above noted objections.